markup: render TeX math as MathML #294

TeX math in markdown and org renders as source.

• Markdown $…$ and $$…$$ (goldmark extension), org LaTeX fragments and \[…\].
• Rendered server-side to MathML, which current browsers display natively; the CSP does not change.
• Needs a Go TeX-to-MathML converter; evaluate what exists before writing one.
• The ugcHTML policy admits the MathML elements and attributes the converter emits and nothing else.
• Applies everywhere markup renders: READMEs, wiki, issues, MRs, comments, releases, snippets.